1. Field of the Invention
The present invention relates to a hydraulic control circuit for automatic transmissions mounted on motor vehicles, particularly to a control circuit suitable for use in multiple speed automatic transmissions by which multiple speeds such as six forward speeds are obtained by combining a sub-transmission unit such as an overdrive mechanism with a main transmission unit such as a three speed transmission mechanism, and more specifically to a hydraulic control device for electronically controlled automatic transmissions equipped with electronically operated regulating valves such as electronically controlled throttle valves.
2. Description of the Prior Art
Automatic transmission in general demonstrate what is commonly known as creeping when a vehicle with the automatic transmission is stopped in D range, where the vehicle tends to move in a forward direction due to the tractive torque delivered by the torque converter. For this reason, a driver of the vehicle must continuously apply pressure on the brake pedal to maintain the vehicle at standstill, which not only increases driver's fatigue, but also increases fuel consumption by subjecting the engine to extra load.
A control device has therefore been proposed previously, wherein the transmission is automatically shifted up to a higher range when the vehicle is stopped, to prevent creeping by the smaller torque transmitted in higher range.
Another device has also been proposed, wherein creeping is prevented by disconnecting the forward clutch when the vehicle is stopped, so that no torque is transmitted to the wheels.
However, while the abovementioned example which shifts up to a higher range is capable of preventing creeping in normal idling condition, creeping still occurs while idling at higher engine revolutions during engine warm-up. Also, improvement in fuel conomy cannot be expected since the engine is still under load.
With the abovementioned example in which the forward clutch is disconnected, in order to avoid shift shock while reconnecting the clutch when starting, special equipments such as accumulators, modulator valves and solenoid valves are required, which results in the complexity and bulk of the unit, and also leads to cost increases and problems in reliability.
Further, with the forward clutch disconnected, the vehicle is liable to start rolling even on slight grades, as the vehicle is no longer restrained from movement. It is therefore desirable for such anti-creep devices to incorporate a hill-holding mechanism for preventing the vehicle from rolling on grades, but the addition of brake and selecting valve necessary for hill-holding leads even more to complexity of the device.